Pulse Width Modulation (PWM) or Class D signal amplification technology has existed for a number of years, but has become more popular with the proliferation of Switched Mode Power Supplies (SMPS). Since this technology emerged, there has been an increased interest in applying PWM techniques in signal amplification applications. This is, at least in part, a result of the significant efficiency improvement that can be realized through the use of Class D power output topology instead of the legacy (linear Class AB) power output topology.
Early attempts to develop signal amplification applications utilized the same approach to amplification that was being used in the early SMPS. More particularly, these attempts utilized analog modulation schemes that resulted in very low performance applications. These applications were very complex and costly to implement. Consequently, these solutions were not widely accepted. Prior art analog implementations of Class D technology have therefore been unable to displace legacy Class AB amplifiers in mainstream amplifier applications.
Recently, digital PWM modulation schemes have surfaced. These schemes use Sigma-Delta modulation techniques to generate the PWM signals used in the newer digital Class D implementations. These digital PWM schemes, however, do little to offset the major barriers to integration of PWM modulators into total amplifier solutions. Class D technology has therefore continued to be unable to displace legacy Class AB amplifiers in mainstream applications.
One of the problems that exists in current PWM amplifier systems is that the processing of audio signals cannot be easily controlled to achieve optimal performance. This problem can be illustrated using the diagram of FIG. 1. FIG. 1 is a block diagram illustrating some of the basic components of a prior art digital PWM amplification channel. As depicted in the figure, the components of amplification channel 100 comprise a noise shaper 110, a modulator 120 and an output stage 130.
High precision PCM Audio data (typically 16 or more bits wide) is input to noise shaper 110, where it is quantized. Typically, the data is quantized to the order of 5-10 bits. The quantized output of noise shaper 110 is then input to modulator 120, which translates the PCM data to pulse-width-modulated (PWM) data. Typically, the data produced by modulator 120 comprises a high-side signal and a low-side signal. These signals are used to drive the high-side and low-side FETs (field effect transistors) of output stage 130, which produces an amplified PWM signal. The signals provided to output stage 130 are typically low pass filtered to remove high frequency noise.
Referring to FIG. 2, a functional block diagram illustrating the structure of a noise shaper in accordance with the prior art is shown. It can be seen from the figure that noise shaper 110 consists of a quantizer 210 and a filter 220. An input data stream consisting of PCM audio data combined with filter data produced by filter 220. The transfer function of filter 220 is designed to filter the difference between the input data stream and the output data stream in order to attenuate the noise created by quantizer 210 in the audio band and amplify the noise at higher frequencies. Quantizer 210 is designed to process the combined data by mapping the data to a discrete number of output values. Quantizer 210 thereby performs a rounding function (rounding the received PCM data to the nearest quantized value) and a clipping function (mapping received PCM data outside the quantized range to either the maximum or minimum values that can be represented)
When the input to quantizer 210 exceeds the quantized range of values, the quantizer clips the data. The resulting output of quantizer 210 is similar to clipping in an ordinary analog amplifier, in which the peaks are removed from the signal. This distorts the audio signal represented by the data and produces audio artifacts that can be audible. The clipping of the signal received by quantizer 210 also results in quantization error. Quantization error is the difference between the input to quantizer 210 and the output from quantizer 210. The quantization error increases when quantizer 210 clips the signal. The quantization error can produce instability in the noise shaper, as well as other undesirable audible effects.
Conventionally, the problems caused by clipping of the signal in the noise shaper are addressed by using a clipping circuit separate from the quantizer to clip the input signal before it is input to the noise shaper. This clipping circuit is configured to clip the signal at levels which are lower than the levels at which the quantizer in the noise shaper clips. While this does relieve the problem of quantization error that would otherwise occur as a result of clipping by the quantizer, it does not eliminate the distortion of the signal from clipping, and may even aggravate the problem, since the clipping circuit clips the signal at lower levels than the quantizer. Put another way, the use of the clipping circuit has the undesirable effect of limiting the maximum range of the output (the modulation index) more than is strictly necessary.